Control system



March .13, 1945. G. A. BROOKE, JR., Erm.

CONTROL sYsTEM Fle-d June 6, 1942 2 Sheets-Sheet 1 SSN Elem March 13, 1945. G, A, BROOKE, JR.,- ErAL 2,371,590

coNTnoL SYSTEM l Filed June e. 19'42 2 sheets-sheet 2 RELAY if .9 I @5 l ma @faz/M70@ 11N 75 i s l l WITNESSES: INVENTORS 'Patented Mer. i3, 1945 UNITEDI STATES PATENT :OFFICE e CONTROL SYSTEM George A. Brooke, Jr., elphla, and Carl C.

Chambers, Lansdowne, Pa., ors to Brooke Engineering Company, Inc., Philadelphia, Pa., 'a corporation oi Pennsylvania l,Application June 6, 1942, Serial No. 446,118

2 2 Claims. VPiCl. 236-15) transmission through theflue gases of the steam generating apparatus to be controlled. The signal impulse from the electric eye' is then amplition in a form especially suited to such use; but

it'will be readily understood by those skilled in the art that, by suitable modifications wit-hin the scope ofthe appended claims, the illustrated embodiment is readily adaptable to other types of apparatus requiring automatic governance, as for example, controls responsive to movements of a, galvanometer, a mechanical or electrical meter,

`or any other sensitive device capable of responding to changes in a condition which it is desired t0 control.

One of the problems encountered in the regulation of steam generating apparatus by automatic control systems of the kind referred to arises from a tendency toward, over-controlling Vand consequent hunting.- I'his is particularly organizations of thisnature, a, further difficulty fled by a control electron tube and balanced against the output from a balance electron tube. By means cf thyratron tubes a reversible motor for actuating the air input to the furnace is then started, its direction and period of operation depending upon vthe predominance and strength of the signal voltage derived from the control tube and the balance tube. Modulation of the outputs of the control tube and the balance tube is ef- "s encountered due to the fact that the density a means for automatically counterbalancing the signal voltage or' impulse so as to modulate the influence of the control means upon a proportional movement of a controlled element; and a fected by changing the grid bias on these tubes. In the case of the control tube., a modulated (in creased or decreased) bias is established upon movement of the fuel regulator ofthe furnace. In the case of the balance tube, a modulating (increased or decreased) bias is setup by delayed action on a movement of the means which operates the air supply damper of the furnace. The changed bias so set up is storedin a reslstance-condenser system and is gradually discharged. When the discharge is complete, the modulatng means ceases t0 function and is thereby automatically fully reset to a normal inactive condition until such time as there is another change in the variable resistance. The extent of bias change is dependent, not upon the position of the movable contacts oi' the ,variable resistance in the bias networks, but rather upon the Y extent of the'r displacement in response to movements of either the fuel regulator or. the air damper motor. 'lhe variable bias resistance opi erated mechanically by the fuel regulator ls so .l the variable resistance actuated mechanically by resetting means for lfully nullifying the effect of f the counterbalancing meansso as te' completely reset the modulating means to normal inactive condition after the cause of the disturbance is overcome or satisfied.

Ashereinafter more fully disclosed, an illustrativeembodiment oi' our invention includes an electric eye which responds to a light beam after the air damper motor, when said-motor rotates so as to increase the supply of air, the bias impressed upon the balance tube operates to decrease the output of the latter, and when the air damp- 1 er motor rotates to decrease. the supply of air, l

Fis. I is a view more or lessldlasranimatically represented, of a steam generating apparatus with a control system therefor conveniently embody- 1 ing our invention.

' pending position of the aix-.now damper Itis changed in" Fig. 2 is a wiring diagram'ofthe system; and

Fig. 3 is a view like Fig. 1 showing a modication of our invention.

Connected to the furnace Il of the heating apparatus in Fig. 1 is the usual fuel line II which supplies iiuent fuel such as oil for heating. Air is introduced intothe furnace Il in the usual way by a blower I2, the input of air being controlled by valve means-in the formkof a damper I3 the position of which is subject to change in response to movements of an electromagnetic actuator herein represented as a motor I4. A similar valve mearlS r damper l5 islocated in the stack j I6, its position being determined by. the'operation of furnace pressure regulator I1. Asv shown. the stack I6 has aligned diametral openings I8 and I9 which allow the passage of light from an'in candescent lamp 2li to a photo-electric cell or electric eye 2|. Current for the operation of lamp 20is supplied from -an A. C. power line `through a voltage regulating transformer 23 (Fig. 1); and adjustments in the intensity of the light aremade by means of variable resistance 22.

The output of photo-electric cell 2| is amplified by means of control tube 26 (shown in detail in Fig. 2) and utilzed'to control the-action of the motor I4. Fuel regulator 25 operates in response to pressure ,changes derived from a steam header response to movements of the motor I4 in a man# ner readily (understood from Fig. 1.

In the network connected to the grid circuit of' balance tube 3|, it 'will' be'noted that a. potentiometer 45 is provided, the' movable arm 46 of which is coupled mechanically with the motor I4,

conventionally shown at 46a in Fig. 1, for automatic operation thereby. l'I'he potentiometer 45 operates in conjunction with a condenser 41 in' vapplying a changed bias voltage to balance tube 3|. Movement of the arm 46 of the potentiometer 45 in response to movement of the motor I4 thus produces a modulating action.v Thelrange within which modulation will take place is deterl mined by means of an adjustable resistance 48 having'a value of 20,000 ohms. A similar potentiometer 49 is provided inthe grid circuit of control tube 26, and the movable arm 50 of said potentiometer is coupled mechanically with the fuel regulator 25 as conventionally shown at 50a in Fig. 2. A condenser 5| of about 8 mfd. is interposed between the movable arm 50 of potentiometer 49 and the grid of control tube 26. The adjustable resistance shown at 52 determine'sthe range within which resistance 49 and condenser 5| operate. An adjustable resistance 53 .of 3

' megohms controls the timerequired for return 9, increased pressure in said header causing the regulator to decrease the amount of fuel being supplied. A

Fig. 2 shows the circuit arrangement for the electric eye 2| as well as the circuits involved in relay 24. The output from electric eye 2|, is rst I amplified by means of a controll electron tube 26 which may be of the 12.15 type. The amplified output from control tube 26 is then fed to one side of the divided primary coil of a transformer 21'.4 The filament circuit of tube 26 is in 'series with va 320.0hm` resistance 26, and the filament circuits ofthe balance tube 3| and the rectifier tube 59; and is further provided'in its grid circuit with a condenser 29 and a resistance 30 which may be 0.01 mfd. and 3 meg; respectively. It is to be noted that the control and balance tubes 26 and 3| are directly polarized by reason of the connection of the mid point of the primary coil of the transformer 21 to one side'of such power line and to the anodes ofk said tubes, the cathodes of said tubes being connected to the other sideof ysaid power line. The balance tube3l, which may also be of the 12J5 type, delivers its output to the I other end of the primary coil of transformer 21.

is provided with acondenser 32 and a resistance 33 having values of about 0.01.mfd. and 3 megohms respectively. The output from the divided grids of a pair of thyratron tubes 34 and 35. The

` sensitivity adjustments shown at 36 and 31 are sistances 33 and 39 which may beof the order of',` 0.5 megohm, and condensers 4l and 4| which may have values of 0.05 mfd. Fixed resistances 42 andy 43 are alsoiinterpo'sed inthe grid circuits of the thyratrons, said resistances being of approximately 0.25v megohm. The filaments of tubes 34.'

are heatedv by current supplied through a I4 will rotate in one` direftion or the other vdeon whether tube 34 or tube35 lires. The

f tor I4 protect said As in the case of control tube 26, balance tube 3| ages; and a i'lxed `secondary coil of the transformer 21 is fed to the l to normal of the bias set up by movement of contact 50 'along the coil of potentiometer= 49 and condenser 5|. Similarly, an adjustable resist-l ance 64 controls the time requiredlfor return to normal of the bias by movement of contact 46 v along the coil of potentiometer and condenser 41. Variablefresistance 55 and fixed resistance 56 of approximately 5000 ohms and 50,000 ohms Ivalue, respectively, are for adjusting the normal bias of control tube 26. Similarly variable and fixed resistances 51 and 56. which may have the same values as in the case of resistances 55 and 56. are-provided for adjusting".` the bias on balance 'tube 3|. The normal bias E.' M. F. for the control and balance tubes 26 and 3| is derivedl through a rectifier tube 5.9 of the Y6 type in conjunction with a illter circuit made up cf resistance 6| and condensers 62 and 53. A-ilxed resistance 64 and a voltage regulator 65 are also provided. Rectifier tube 59 also supplies the bias E M. F. for the thyratron tubes 34 and 35 throughl adjustable and fixed resistances 61 and condenser 68.

Limit switches 69 and 69a respectively located at opposite ends of the divided field coil 10 of mowlien its movement in one directionpasses beyond a predeterminedpoint. Fixed resistances shown at 1 l and 12 assure feed-back oi'A stray voltresistance v13 offabout 3000 ohms in value, is relied upon for feeding voltage back' to the momentarily inoperatlve'side of the basis and willoperate iny accordance nal voltage applied tothe thyratronA tubes..` When' 'thyratron circuit.

that thyraton tube which is'not in cperationl The-switch'arrangement shown at 14 allows of f various optional operations. When contact is vmadefwith point 15 the relay system` is rendered inoperative. WhenA contact 'is made 'with point 19 the relay system is 'placed on an lautomatic with the 51g-,

, contact is made withpoint 11'the system will optransformer 44 from the power line, and motor crate to increase the airy supplypand `when contact is made vwith point 16'the systeniwill op-v l'crate toV decrease the air supply.

The system is designed toreduce the of air when the output fromcontrol tube 26 predominates bal- 66 and y motor by de-energizing it thus preventing flickering of y resistances 36 and 3l.

' the blowerwhich supplies forced draft air to ance tube 2|. Since the control and balance tubes 26 and21 are both biased by the same di'- rect current grid voltage and are similar tubes,

the current flow through both the upper and lower halves of the primary of the transformer 2l will be identical, and since they are out of phase will give rise to zero voltage normally in the secondary coil of said transformer. However, i

transformer to the resistances 36 and 3'l. By

choosing the ratio properly, the voltage ratio may be made to appear at the time when the plate voltage of the thyratrons is becoming positive.

Y fired at the beginning of their conductive cycles.

With this in mind it will hev This form of voltage insures that the thyratrons will be seen that a decrease in the output of control tube 26 (assuming a constant output from balance tube 3|) will result in an increased air f supply. When motor I4 operates to increase the air supply, the position of the operatingV arm y46 of potentiometer 45 is changed, condenser 41 is charged proportionally and a bias is set up in the grid circuit of balance tube 3|. This results in a relative decrease in the effect of balance tube 3| and a consequent tendency to rebalance the output of the two tubes 26 and 3|. preventing further immediate movement of motor 4. However, the condenser charge will leak olf through adjustable resistor 54, resistor 51 and 35, and contact 46, lthe time required for said charge to leak off being adjustable by means of resistor 54.

In the c-ase of a movement of the fuel reguthe furnace. In apparatus of this type it is desirable to regulate the volume of the air supply Such an organization is delinto the turbine. cated in Fig. 3, where the air blower I2 is directly connected to the turbine 80 to which steam under pressure is conducted through a pipe 8|. As shown a valve 82 interposed in the pipe 8| is arranged `for actuation by an electro-magnetic actuating unit' |4b (like the unit I4 of Fig. l), whereto the shiftable point of the potentiometer embodied in the relay 24 is mechanically coupled as conventionally indicated at 46h. The shiftable point 50 of the potentiometer 49, on theother hand, is here mechanically coupled as at 50h with a pressure responsive diaphragm4 device 83 in communication with the delivery trunk ofthe blower by way of a pipe 84. By means of our improved control system in- Fig. 3,

'quick response is obtained as a consequence of opening 'of the valve 82 an excessive amount, and, after the air pressure starts to change when the turbine 80 picks up speed, the diaphragm 83 operates to effect, through its influence in the .modulation of the system, return of the valve to the position required for maintaining a new rate of speed of the. turbine.

Having thus described our invention, we claim: 1. A control system for apparatus with a-normally quiescent governing element subject to operation by an electron-magnetic actuator, said .system including means influenced by changes in the behavior of the apparatus for producing an electric, signal impulse; a control electron tube for receiving and amplifying said impulse; means influenced by the voutput of the control tube for supplying current for the operation of the lator 25, an increasing mechanical movement of the arm 50 of potentiometer 49 is made. When the movement of fuel regulator 25 has been such as to increase the fuel being supplied, the ef.

fect of the bias set up in the grid circuit of control tube 26 is in the direction of decreasing its output.' resulting in an increase in the air supply. Increases in the fuel supply are therefore attended by increases in the air supply to the furnace. A change in fuel regulator 25 in the direction of decreased fuel supply, on the other hand, results in a grid bias on control tube 26 which increases its output with resultant decrease in the amount of air being supplied.

It will thus be seen that an increase of fuel will result momentarily at least in an increase in air supply, but this increase will be checked through the mechanical linkage with the mo'tor I4 and potentiometer 45, producing a decreased output in balance tube 3|, thereby re-balancing the two tubes 3| and 26, and preventing a contined oversupply of air. Furthermore, bias caused by charge of condenser 5| and that caused by charge of condenser 41 will disappear as these condensers discharge through' their' respective loop circuits.

. By suitable modifications within the scope of certain of the appended claims, the control system of our invention is also applicable. with attainment of advantages comparable to those hereinbefore pointed out, to steam generating l apparatus where a steam turbine is used to drive electro-magnetic actuator; a biasing circuit for the tube grid with an interposed potentiometer;

'a mechanical connection operated from a movable part of the apparatus other than the aforesaid governing element for regulating the potentiometer to modulate the grid bias; and means for automatically fully resetting the biasing circuit to normal inactive condition upon the lapse of `a definite time interval after initiation of the biasing circuit.

2. A control system for apparatus with a normally quiescent governing element subject to I operation by an electro-magnetic actuator, said system including means influenced by changes inthe behavior of the apparatus for producing an electric signal impulse; a control electron tube for receiving and amplifying said impulse; means influenced by the action of the control tube for supplying current for the operation of the electro-magnetic actuator; a biasing circuit for the tube grid with an interposed potentiometer: a mechanical connection operated from a movable part of the apparatus for regulating the potentiometer to modulate the grid bias;` and a reset means comprising a resistance-condenser network connectedwith the grid circuit of the tube. said .network having a denite discharge period duringr which the biasing circuit is automatically fully reset to normal inactive condition after initiation of the biasing circuit.

3. A control system for apparatus with a normally quiescent governing element subiect to operation by an electro-magnetic actuator, said sys-- tem including means influenced by changes in the behavior of the aoparatus forproducing an elect-ric signal impulse: a control electron tube forreceiving and amplifying said impulse; means influenced by the act-lon of the tube for supplytime of its discharge.

.ing currentv for the operation of the electro-magnetic actuator a biasing circuit for the tube grid with an interposed potentiometer; a mechanical connection operated from a movable part of the apparatus other than the magnetic actuator forregulating the potentiometer in the grid circuit to modulate the action of the tube; and a resetting means comprising a resistance-condenser networkconnected with the grid circuit of the tube, said network having a denite discharge vperiod during which the biasing circuit is automatically fully reset to normal inactive condiment of fthe separate means aforesaid through which the grid bias of the control tube is regudenser networks in circuit with the grids of the tion after initiation lof the biasing circuit, said elements, one subject to operation by a reversil ble electro-magnetic actuator and the other subject tooperation by a separate actuator, said system including means inuenced by changes' in the behavior of the apparatusfor producing an electric signal impulse; an electron control` tube for. Vamplifying said impulse; another electron tube for balancing the control tube; a pair of thyratron tubes for controlling current supply to the magnetic actuator; means in circuit with the grids of the thyratrons and subject to the predominance in the output of the control tube or the balance tube to determine the direction. of movement of the electro-magnetic actuator;

variable bias means for the grids of the'control and balance tubes; modulating means operated a through mechanical connectionswith the two actuators respectively for regulating the bias means of the balance and control tubes; and coordinated resistance-condenser networks respectively in circuit with the grids of the control and balance tubes, said networks having predetermined discharge.- periods whereby the bias circuits are automatically fully reset to normal inactive condition after the initiation of the biasing circuits.

.8. A control system according to claim 5, in which the resistance-condenser networks are in'- dependently adjustable for variation of their respective discharge periods.

'7. A control system according to claim 5, in which the resistance-condenser networks are independently adjustable to vary the extent of bias change upon a given movement of the control means.

8. A control system for apparatus having two` lated to inuence said tube; and resistance-conrespective tubes, said networks' having definite vdischarge periods whereby the modulated bias circuits are .automatically fully reset to inactive condition upon the expiration of -a definite time interval after initiation of the biasing circuits.

9. A combustion control system for steamv generating apparatus having a furnace, regulating means for respectively controlling combustion air and fuel supply to the furnace, an electro-magnetic actuator for the air supply regulator and a separate actuating means' for the fuel supply regulator, said system including means sensitive to changes in internal conditions of the furnace for producing an electric signal impulse; control means aiTected by such impulse for operating the actuator for the combustion air regulator; modulating means automatically actuated through connections with the lair regulator and fuel regulator actuators for influencing the control means;

and means cooperating to automatically fully reset the modulating means to normal inactive condition upon the expiration of a definite time interval after initiation of the modulating means.

10. A c ombustion control'system for steam generating' apparatus having a furnace, means for respectively regulating the supplyof combustion air and fuel to the furnace, an electromagnetic actuator for the air regulator means,

and a separate actuator for the fuel supply regulator means, said system including a light source for projecting a 'light beam through the combustion gases passing from the furnace; a photo# electric cell influenced by variations in the in'- tensity of the light beam; a voltage sensitive electric control element responsive to the action of the photo-electric cell for governing the operation of the actuator for the air regulator; modulating means in circuit with said control element ,and subject to governance mechanically by the actuator for the fuel regulator to influence the control element; and resetting means for automatically fully resetting the modulating means to normal inactive condition upon expiration of normally quiescent governing elements. one subject to'operation by an electro-magnetic actuator, and the other subject to operation by a separate means, said system including a control elec-5 tr'on tube; an opposing balance tube whose outl' put normallybalances that1 of the control tube;

means Vinfluenced by predominance inthe output from one or the other of the two tubes for moving the electro-magnetic actuator in one directionv a definite time interval .after initiation of the modulating means.

l1. A combustion control system for steam generating apparatus having a furnace, regulating means for respectively controlling the supply of combustion air and fuel to the furnace, an electromagnetic actuator for the air regulating means, and a separate actuator for they fuel supply regulating means, said system including alight source for projecting a light beam through the combustion gases passing from the furnace; a photoelectric cell influenced by variations in the intensity of the light beam; a voltage sensitive control element responsive to the photo-electric cell for governing the operation of the air supply regulating means; modulating means in circuit with the control element and subject to mechanical operation respectively by the fuel regulating means for influencing `the action of said controlv element; and resetting means for fully resetting the modulating 4means to normal inactivecon'dition at the expiration of a definite time interval after initiation of the modulating means.`

l2. A combustion control system for steam generating apparatus having a furnace, means for respectively regulating thesupply of combustion air and fuel to the furnace, an electro-magnetic actuator for the air regulating means. and a septhyratron tubes for I asvipoo supply regulator operated through changes in steam pressure, said system including Ia control electron tube; a balance tube whoseoutput is balancing the output from the control tube; a

pair of thyiatron tubes for governing directional operation of the air regulator actuator in accordance with the strength of the impulsederived from the control tube or the balance tube; vari;

able grid biasing means respectively for the control and balance tubes; modulating means automatically operated throughmechanical conne'c tions with the actuators for the air and fuel regulators for varying the grid biasing means respectively of the balance and control tubes to govern the system; and resetting means for fully restoring said modulating means to normal inactive condition upon a lapse of a predetermined time interval after initiation of the biasing means.

13. A combustion control system for steam genthe. furnace of the generator-,for producing an balanced against the output of the control tube;

meansl influenced by predominancein the output from one or the other of the two tubes for moving the electro-magnetic actuator inione direction or the other; means influenced -by changes in the density of the combustion products within electric signal impulse to affect the grid voltage of the control tube; variable bias means respectively for the grids oi! the control and balance tubes; a mechanical connection through. which the grid bias means of the balance tube is modulated to vary' the effect of said tube upon movementiof the electro-magnetic actuator; a mel chanical connection through which the 'grid bias erating apparatus .having a furnace, means for respectively regulating the supply of 'combustion air and fuel to the furnace, a reversible electric motor for actuating the air regulating means, and a separate means for actuating thefuel regulatins producing an electric signal impulse; a control electron tube for amplifying the signal impulse; a balance electron tube for balancing the output of the control tube: a pair of thyratron tubes for governing the direction and the extent of opera- Vtion'of the air valve actuating motor in accordance with predominance of output from the control tube or the balance tube; variable grid biasing means respectively. for the control and balance tubes; means automatically operative through mechanical connections respectively with` the actuators for the air and fuel regulating means for varying the grid biasing means of the balance and control tubes to modulate their outA puts; and resetting means automatically operative to fully reset the biasing means to normal inactive condition upon the ,lapse of a dennite \time interval after initiation of the biasing means.

14. A combustion control system for steam gen-Y erating apparatus having a furnace, means for respectively regulating the supply of combustion air and fuel to the furnace, an electro-magnetic means, said system includingmeans sensitive yto changesin conditions within the furnace for means of the control tube is regulated upon move-I ment of the fuel degulator of the furnace to actuator therefor. and a normally quiescent fuel.

, modulate the intluence'of the latter tube; and resetting means comprising `resistence-condenser networks in circuit with the grids of the respective tubes, said networks having definite discharge periods whereby the grid biasing means are automatically fully restored to normal inactive con-` dition upon the expiration of 'a `definite time ini terval after the initiation of the biasing means.

16. A combustion control system for 'steam generating apparatus embodying a furnacaregulating means for governing the supply of combustion air to the furnace, and an electrically-controlled actuator for the regulating means. said system including means influenced bychanges in the behavior of the apparatus for producing an electric signal impulse: a control electron tube for receiving and amplifying such impulse; means indu actuator for the air regulating means, and a sep` l arate means for actuating the fuel regulating means, said system including means sensitivev to changes in conditions within tl1e""furnace for producing an electric signal impulse: a control electron'tube for amplifying said impulse; a balance electron tube for balancing the controltube;

governing directional operafor the airregulator in accordance with the predominance of the output of the control tube or the balance tube; variable grid biasing means respectively for the control and tion of the actuator balance tubes; mechanical connections for respectively actuating the grid biasing means of the talance and control tubes for the air and fuel valve actuators to. modulate the influence of said tubes; and resetting means for automatically fully resetting the grid biasing means tb inactive normal lcondition upon the expiration of a deilnite time interval after initiation of the biasing means. i 15. V-A control system for a steam generating,

apparatus having a furnace; a normally quiescent air supply regulator with an electro-magnetic from the 'actuators' enced by the output of the control tube for supplying current for the operation of the actuator aforesaid; a biasing circuit for the tube-grid with an interposed potentiometer; a mechanical connection operated from a movable part of the apparatus other than the aforesaid regulating means for adjusting the potentiometer to modulate the grid bias; and means for automatically fully resetting the biasing circuit 'to normal inactive condition 4upon the4 lapse of a denlte time interval after initiation of the biasing circuit.

17. The invention according to claim 16, in

which the movable part of the' apparatus is in the form of aregulator for'governing the supply of fuel to the furnace.

18. A combustion control system for steam generating apparatus embodying a furnace,`regu lating-.means for governing, supply of combustion air to the furnace, 'and an electrically-controlled actuator for the regulating means. said system including means sensitive to density. changes in the flue gases passing from the furnace for producingan electric signal impulse; a control electron tube lfor receiving 'and ampliflying such impulse; means influenced by the output of the control tube for supplying current to operate the magnetic actuator aforesaid; a biasing circuit for the tube grid with an interposed potentiometer:

.a mechanical connection operated from a movable part of the apparatus other than the aforesaid regulating means for adjusting the poten- 19. A combuston control systeml for steam generating apparatus embodyingl a furnace, a turbinefdriven blower for injecting air into the furnace, regulating` means for. governing supply of steam to the turbine, and an electrically-com' trolled actuator'for the regulating means, said system including means sensitive to changes in' the internal condition of the furnace for progrid with an interposed potentiometer; a' rmechanical connection operated from a movable part of the apparatus other than the aforesaid vregulating means for adjusting the potentiometer to modulate the grid bias; and automatic means for fully resetting the biasing circuit to normal inactive condition upon the lapse of a definite time interval after initiation of the biasing circuit.

20. A combustion control system for steam generating apparatus embodying a furnace. a

n turbine-driven blower for supplying combustionsupporting air to the furnace.' valve means for 'governing the supply of steam'to the turbine,

and an electrically-controlled actuator for the` valve, said system including means sensitive -to changes in the internal condition of the furnace u for producing an electric signal impulse; a control electron tube for receiving and amplifying such impulse; a biasing circuit for thetube grid with an interposed potentiometer; means iniluenced by the output of the control tube for supplying currentv for operating the actuator for 85 the valve means;` means respectively coupled mechanically with the valve actuator and a device sensitive to pressure change vin the delivery end of the blower for modulating the grid. bias of the tube; and means for automatically fully 40` in accordance with the physical characteristic of the means under control; a pairof thyratron resetting the biasing circuit to normal inactive condition upon the lapse of a dennite time interval after initiation of the biasing circuit.

21. A control system for apparatus with two normally quiescent inter-dependent governing 45 elements, one subject to operation bya reversible electro-magnetic actuator, and the other subject to operation bya separate actuator, said 'system including means influenced by changes in the behavior of the apparatus for producing an electric signal impulse;'an electron tube for amplifying such impulse; another electron tube for balancinglthe vcontrol tube; a transformer with' a divided primary coil themid point of which is connected to a source of alternatingcurrent,

and to the opposite ends of which the .anodesof said control and balance tubes are respectively connected; a pair of thyratron tubes with their grids connected respectively to opposite end sections of a divided secondary coil` of the trans-` former; means in circuit with the grids of the thyratrons and subject to the predominance in means operated through mechanical connectionsy with the two actuators respectively for regulating the bias means for the balance and control tubes; and coordinated resistance-ccndenser networks respectively in circuit with the grids of the control and balance tubes, said networks having predetermined discharge periods whereby the biascircuits are automatically fully reset to normal inactive Vcondition afterv each operation of the system.

22. In an electronic control system for controlling ymeans subject to changes in physicaly characteristics, a transformer having primary and secondary coils with the mid point of the primary coil connected to the one side of an A. C. power line; a divided resistance connected across the endsof the secondary coil of the transformer; a pair of vacuum tubes arranged in opposed relation and having their anodes connected respectively tothe yopposite ends of the primary coil of the transformer and lwith their cathodes -connected to the other side of the A. C.

power line; means normally applying D. C. blas.

ing voltage in'like degree to the grids of the vacuum tubes; means for unbalancing said grids `sistance; and means energized through said thyratron -tubes to effect a correction in the physical characteristic of the means under con- I trol.

' GEORGE A. BROOKE, Ja.

. CARL C.- 

